Preparation of the refractory metals by fused salt electrolysis



PREPARATION OF'THE REFRACTORY METALS BY FUSED SALT ELECTROLYSIS Menahem MerlnbQSohel and Milton J. Arnolf, Cleveland,

and Jack L; Sorkin, Cleveland vl leig'hts, Ohio, assignol s,

hymesne assignments, to Horizons Titanium Corporatron; Princeton, N. 1., a'corporation of New Jersey No Drawing. Application: Novembers8', 1955 s rial N0.- 545,775

12 Claims, (Cl..2.04--64)' This invention relatesito the preparation of the rare and refractory metals of the fourth, him and sixth groups of the periodictable, havingan atomic number between 23 and 74, that is, the metals vanadium, chromium, zirconium, columbium, molybdenum, hafnium, tantalum and tungsten. More particularlyit relates to the preparation of fused salt compositions adapted to be electrolyzed to produce these elements in the form of a cathode deposit and to the electrolysis of such fused salt compositionsf Numerous processes have been suggestedfor the preparation of the refractory metals. One type of process which was exhaustively investigated is described in U. S. Patents 1,815,054and 1,835,025 issued'to Driggs and others. The processesth'erein described and claimed comprised electrolytically' depositing a desired refractory metal from a fusion of an alkali metal halide and a double halide compound of an alkali metal and the rarerefractorymetal. As taught in these patents, the double halidewas preferably a fluoride. The processes were disclosed as being applicable to such metals as tantalum, tungsten, manganese, zirconium, thorium and to members of the samev groups as these metals.

Instead of employing a fused bath in which the desired metal is present as a double fiuoride, we have found that a much simpler and more economical procedure may be effected through the electrolysis of anhydrous chlorides of'the metals in question ina fused salt bath composed essentially of at least one alkali metal halide or alkaline earth metalhalide. In accordance with our invention, an oxygen-free, anhydrous, chloride-containing bath may be prepared from such common and relatively inexpensive raw materials as oxygen-containing compounds of the metals. Byoxygen-containing compounds we intend to include those compounds which, onheating to temperaturesno higher than about 1000 C., and particularly when dispersed in a fused salt, form the oxide of the metal in'nascent, or reactive form. Among such compounds may be mentioned the hydroxides and hydrated oxides, nitrates, .nitrites, oxal'ates, acetates; carbonates, basic carbonates and other basic salts such as basic chlorides or nitrates, and hydrates which decompose to form the reactive oxide by hydrolysis. It is the fundamental concept of our invention that the oxide of the metal to be converted to the chloride shall be available to the chlorination reaction in nascent or reactive form, it being long known that time-aged or highly-ignited metal oxides are relatively refractory to chlorination. In our invention, not only is the oxide in freshly-prepared reactive form, but it is also highly dispersed in the fused alkali salt, therebyaffording excellent physical conditions for conversion to chloride. 1

Briefly, in accordance with our process, such oxygen containing compounds are added, either singly or in combination with one another, to a molten pool comprised of 'atleast one alkali metal halide and preferably at least one alkali metal chloride. When the addition is made, a dispersion or possibly a partial solution of the rare and refractory metal oxide is forrnedin the fused bath while at thcl same time, any water present flashes off at the v uously additional amounts of refractorymetal compound are added periodically'to thefused bath in the reactor" and the alkali metal chloride content of the bath 'in'the-r hydrogen chloride a o tiu d'r qr r31 il iitill i ice elevated temperature of thebath; We have found that when this solution or dispersion is contacted with a suitable chlorinating agent, a reaction occurs in which'the desired metal chloride forms in an anhydrous condition in the fused salt bath. In the past when the desired metal chloride has been independently formed and then added to a fused bath, during the interval between formation and addition, the chloridewas'highly susceptible to hydrolysis and hence pickedup a considerable amount of moisture. By our process this interval is eliminated completelyand a. virtually 'oxygen-free bath of the chloride is produced.

The chlorination reaction. may be effected-with a wide variety of chlorinating, agents including anhydrous hyadrogen chloride, phosgene, thionyluchloride, mixtures-of:

chlorine with carbon monoxide and mixturesiof chlorine with carbon. The. chlorinating agent maybe introduced into the fused salt melt in any suitable fashion. We have 1 chlorinated with finely divided bubbles, passed. into the melt through an-inert perforated inlet tube. With ap? propriate starting 'materialsthe' chlorinationproceeds quantitatively and, depending on thehchlorinating agent selected, either water vapor or various oxides of carbon As sh'own'in the foregoing'reacti-ons anywater-vapor and carbon oxides formed as a result of the reaction are readily eliminated from the'melt. Accordingly, thefused mass may be electrolyzed directly in the vessel in which the reaction was effected. We prefer, however, to'operate the reactor continuously. Accordingly, we with-- draw portions of the reaction composition and charge the molten material into anelectrolytic cell constructed of a suitably inert material, wherein the molten material is electrolyzed employing a caroonuor graphite-anode in:

a manner well known in the art. Chlorine is evolved at the anode and the desiredrefractory metal is deposited at the cathode. To enable thereact-or torrun continreactor is replenished from time to time.

The following examples 'willserve to further illustrate the practice of our invention.-

Example I A mixture of. sodium chloride and potassium chloride was fused in a covered graphite -crucible ataatemperature between 700 and 725 C. The chlorides werejpropiontioned to form a eutectic composition. After ,thesalts added to the bath slowly over a period of ten-minutes. Due to the rapid release of water vapor and fumes containing oxides of nitrogen, the additions-Were small ginsize and were made gradually; Aboutgtwenty minutes after the final increment of nitrate had beenae dded the bath had become a transparent red color, Th

Example {I The experiment of Example I was repeated, adding a tantalum salt instead of the chromium salt. The tantalum salt added was the hydrated pentoxide produced by acidification of an aqueous solution of potassium hexatantalate. The alkali metal hexatantalates, per se, may be added to the fused salt melt in an entirely similar fashion. The tantalum chloride evolved from the fused melt as rapidly as it formed and was recovered and separated from other gaseous products leaving the cell, by passing it into a large body of molten alkali metal chlorides, wherein it dissolved or formed complexes. From such a bath the metal could be readily recovered by electrolysis.

Example III Into a fused salt bath prepared by melting sodium chloride in a graphite crucible, there was introduced a stream of chlorine gas. The gas was bubbled into the melt by means of a perforated graphite inlet tube. After the flow of chlorine was initiated, pellets of basic zirconium carbonate and lampblack carbon, intimately admixed in the proportions of parts of basic zirconium carbonate containing about 23.4% ZrO by weight to 1 part by weight of lampblack were gradually fed into the molten bath. The melt was heated to about 800 C. and maintained at this temperature during the chlorination. A sample of the salt bath was taken after completion of chlorination and analyzed for zirconium. It was found that the fused melt contained 2.5% zirconium corresponding to 6.4% ZrCl, by weight.

The fused salt was then electrolyzed by inserting an anode and a cathode through openings provided in the cover of the crucible. The electrolysis, carried out at 750 C. under an atmosphere of argon at a current density of about 100 amperes/square decimeter, and a cell voltage of about 2.3 volts produced a deposit of zirconium metal on the steel cathode.

While the foregoing examples illustrate a preferred manner of adding the transition metal oxygen containing compound to a molten salt bath, it is advantageous in many instances to melt the oxygen containing compound and the carrier salt at the same time and then to chlorinate the resulting mixture. Accordingly, it is not our intention to limit the following claims to any particular manner of forming the fused reaction bath, provided that the metal oxygen compound is present therein in reactive form.

The transition metal chlorides produced in accordance with our invention may be separated from the fused salt melt by any suitable technique, such as extraction by an organic solvent, or, as we prefer, they may be further processed to obtain the metal itself, e. g., by electrolysis of the fused bath or by reduction with a powerful reducing agent such as an alkali or alkaline earth metal (e. g. sodium).

We claim:

l. The process of preparing an anhydrous chloride of a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused salt melt consisting of at least one alkali metal halide, adding to the melt a compound of said metal selected from the .4 oxygen containing compounds of said metal which decompose to yield an oxide of. said metal in said melt, and chlorinating said freshly formed metal oxide contained in the resulting melt by admitting an externally produced chlorinating agent into the melt wherein it converts the freshly formed oxide into an anhydrous chloride ofthe metal.

2. The process of preparing an anhydrous chloride of a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused salt melt consisting of at least one, adding to the alkali metal halide melt a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal in said melt and chlorinating the freshly formed metal oxide contained in the resulting melt by admitting an externally produced chlorinating agent of'the group consisting of chlorine, phosgene, thionyl chloride, hydrogen chloride, mixtures of carbon monoxide and chlorine and mixtures of carbon and chlorine into the melt wherein it converts the freshly formed oxide into an anhydrous chloride of the metal.

3. The process of preparing an anhydrous chloride of a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused melt of sodium chloride, adding to the molten sodium chloride a compound of said metal selected from the oxygen con taining compounds of said metal which decompose to yield an oxide of said metal and chlorinating the freshly formed metal oxide contained in the salt melt by admitting a chlorinating agent into the melt wherein it converts the freshly formed oxide into an anhydrous chloride of the metal.

4. The process of preparing an anhydrous chloride of a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused salt melt consisting of an alkali metal halide, adding to the melt a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal in said melt, chlorinating the freshly formed metal oxide contained in the salt melt by admitting an externally produced chlorinating agent thereinto and recovering the resulting anhydrous chloride of said metal in solution in a fused alkali metal halide.

5. The process of preparing an anhydrous chloride of zirconium, which comprises: preparing a fused salt melt consisting of alkali metal halide, adding to the alkali metal halide a compound of zirconium selected from the oxygen containing compounds of zirconium which decompose to yield zirconium oxide in said melt, and chlorinating the freshly formed zirconium oxide contained in the melt by admitting an externally produced chlorinating agent thereinto, whereby an anhydrous chloride of zirconium is produced.

6. The process of preparing an anhydrous chloride of chromium, which comprises: preparing a fused salt melt consisting of alkali metal halide, adding to the alkali metal halide a compound of chromium selected from the oxygen containing compounds of chromium which decompose to yield chromium oxide in said melt and chlorinating the freshly formed chromium oxide contained in the melt by admitting an externally produced chlorinating agent thereinto. whereby an anhydrous chloride of chromium is produced.

7. The process of preparing a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused melt consisting of at least one alkali metal halide, adding to the alkali metal halide melt a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal dispersed in said melt, chlori-,

nating the metal oxide contained in the salt melt by admitting a chlorinating agent thereinto thereby producing an anhydrous chloride of said metal, and recovering the said metal from said chloride.

8. The process of preparing a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused melt consisting of at least one alkali metal halide, adding to the alkali metal halide melt a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal in said melt, chlorinating the metal oxide contained in the salt melt by admitting a chlorinating agent thereinto thereby producing an anhydrous chloride of said metal in solution in said fused salt melt and electrolyzing the fused solution to obtain said metal as a cathode deposit.

9. The process of preparing a metal of the group consisting of vanadium, niobium, tantalum, zirconium, hafnium, chromium, molybdenum and tungsten, which comprises: preparing a fused melt of sodium chloride, adding to the molten sodium chloride a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal in said melt, chlorinating the freshly formed metal oxide contained in the melt by admitting into the melt a chlorinating agent of the group consisting of chloride, phosgene, thionyl chloride, hydrogen chloride, mixtures of carbon monoxide and chlorine and mixtures of carbon and chlorine, thereby producing an anhydrous chloride of said comprises: preparing a fused melt of sodium chloride, adding to the molten sodium chloride a compound of said metal selected from the oxygen containing compounds of said metal which decompose to yield an oxide of said metal in said melt, chlorinating the metal oxide contained in the melt by admitting a chlorinating agent thereinto, thereby producing an anhydrous chloride of said metal in solution in said fused melt and recovering said metal from said dissolved anhydrous chloride.

11. The process of preparing zirconium which comprises preparing a fused melt consisting of at least one alkali metal chloride, adding a zirconium carbonate to the molten chloride, chlorinating the resulting dispersion of zirconium oxide in the melt by admitting a chlorinating agent thereinto, thereby producing an anhydrous zirconium chloride in solution in said alkali metal chloride and recovering the Zirconium therefrom.

12. The process of preparing chromium which comprises preparing a fused melt consisting of at least one alkali metal chloride, adding a nitrate of chromium to the molten chloride, chlorinating the resulting dispersion of chromium oxide in the melt by admitting a chlorinating agent thereinto, thereby producing an anhydrous chloride of chromium in solution in said alkali metal chloride and recovering the chromium therefrom.

References Cited in the file of this patent UNITED STATES PATENTS Australia Apr. 8, 19 54 UNITED STATES P E T FICE. CERTIFICATE OF CORRECTION Patent No, 2,870,073 I matey-20 1959 Menahem MerlxibqSobel et al. I

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters .Patent should read as corrected below.

Column 4, lines 12 and 13, strike out "alkali metal halide? and insert the same after "one' and before the coma, in line 12; same column.

Signed and seeled this 19th day oi May 1959.

(SEAL) Attest:

KARL MINE ROBERT c. WATSON Attesting Officer 7 Commissioner of Patents 

1. THE PROCESS OF PREPARING AN ANHYDROUS CHLORIDE OF A METAL OF THE GROUP CONSISTING OF VANADIUM, NIOBIUM, TANTALUM, ZIRCONIUM, HAFNUIM, CHROMIUM, MOLYBDENUM AND TUNGSTEN, WHICH COMPRISES: PREPARING A FUSED SALT MELT CONSISTING OF AT LEAST ONE ALKALI METAL HALIDE, ADDING TO THE MELT A COMPOUND OF SAID METAL SELECTED FROM THE OXYGEN CONTAINING COMPOUNDS OF SAID METAL WHICH DECOMPOSE TO YIELD AN OXIDE OF SAID METAL IN SAID MELT, AND CHLORINATING SAID FRESHLY FORMED METAL OXIDE CONTAINED IN THE RESULTING MELT BY ADMITTING AN EXTERNALLY PRODUCED CHLORINATING AGENT INTO THE MELT WHERIN IT CONVERTS THE FRESHLY FORMED OXIDE INTO AN ANHYDROUS CHLORIDE OF THE METAL. 